There has long been a need to quantify particle concentration and to verify particle concentration in cylinder gases. While acceptable levels of particle content in cylinder gases continue to be assessed within SEMI and elsewhere, the predominant issue is the selection of an appropriate method for obtaining accurate, meaningful data.
Significant effort has been expended in establishing appropriate techniques for sampling particles from bulk gas pipelines for which particle specifications generally exist. However, the true particle content of compressed cylinder gases is more difficult to define for several reasons. Firstly, full cylinder pressure is typically 20 times greater than that of a pipeline, making pressure reduction for particle sampling a much more difficult task. Furthermore, the pressure in a gas cylinder, as opposed to a pipeline, decreases with usage which affects the detected particles in many ways. As a result, sampling techniques employed for pipeline gases are not directly applicable to cylinder gases. There are sampling artifacts associated with cylinder gas pressure reduction. For example, a pressure regulator which is universally employed is a source for small particles and a sink for large particles. The paper entitled Factors Affecting Particle Content in High-Pressure Cylinder Gases authored by Drs. Wang, Wen and Kasper of American Air Liquide taught the use of a particle analysis system for compressed cylinder gases which consisted of a means for pressure reduction followed by two particle counters in parallel. A laser particle counter model LAS-X from PMS and a condensation nucleus counter model 3760 from TSI were employed. However, there remained a number of practical obstacles which, prior to the present invention, prevented a straightforward, non-complex method of performing impurity analysis for compressed gases.
Recently, particle sensors capable of counting particles under a high pressure environment (up to 3,000 psi) became available (PMS-CGS, HIAC-ROYCO 5400). This eliminates the need for pressure reduction and its associated problems. However, using these instruments introduces the additional problem of gas introduction and flow control. Regarding gas introduction, direct introduction of cylinder gases into an impurity sensor involves an initial pressure pulse in the range of 100-3,000 psi. Contaminants generated by the initial pressure pulse often contaminate the impurity sensor and disable its operation. There is a need for a method to prevent the contamination caused by the initial pressure transient. On the subject of flow control, gas cylinders contain a limited amount of gas; cylinder pressure decreases with gas consumption. For a sensor requiring constant residence time, continuous adjustment of the mass flow controller is necessary to accommodate changes in cylinder pressure. There is a need for a device which maintains constant volumetric flow for gases of varying pressure.
Further, many impurity sensors require a constant residence time in the sensing volume of the instrument to make an accurate measurement. In other words, it requires a constant volumetric flow rate, independent of its operating pressure. Flow control for this type of sensor is trivial if the sensor is operated at a fixed pressure, since a simple conversion factor can be used to convert mass flow rate to volumetric flow rate. However, if the pressure of the gas sample varies with time, which is characteristic of cylinder gases, continuous adjustment is required for commonly used flow control devices such as mass flow controllers or rotameters.
It is thus an object of the present invention to provide a system to determine true particle content of compressed cylinder gases in a straightforward reducible and effective manner.